Pigments generally exhibit vivid color tone and high coloring power, and they are widely used in many fields. Applications of pigments include, for example, coatings, printing ink, electrophotographic toner, ink for inkjet printers and color filters, and pigments have now become an important compound indispensable in everyday life.
General properties and classification of applications of pigments are described, for example, in Non-patent Document 1. Of the aforementioned applications, ink pigments for inkjet printers and pigments for color filters require high performance and are practically particularly important.
Although dyes have been used as a coloring material for ink for inkjet printers, they have disadvantages in water resistance and light resistance. To improve such problems, pigments are now being used. Images obtained using pigment ink have a special advantage that their light resistance and water resistance are better than those of images obtained by dye ink. However, formation of uniform fine particles of pigment ink of a nanometer size which can infiltrate through spaces on the surface of paper (i.e., monodispersing) is difficult, and thus there is a problem that the pigment ink has poor adhesion to paper.
With an increase in the number of pixels in digital cameras, thinning of color filters used in CCD sensors is in demand. Organic pigments are used in such color filters, and as the thickness of the filter depends largely on the particle size of the organic pigment, there has been a need to produce monodisperse and stable fine particles of a nanometer size level.
Generally, methods of producing fine particles are roughly classified into breakdown methods in which fine particles are produced from a bulk material by pulverization or the like as described, for example, in Non-patent Document 2 and build-up methods in which fine particles are produced by particle growth in a gas phase or liquid phase. Although the method of producing fine particles by pulverization based on a breakdown method has been frequently used and is highly practical, it involves various problems such as extremely low productivity for producing particles of organic materials of a nanometer size and limitation on materials to which the method can be applied. In this situation, formation of nanometer-scale fine particles of organic materials by a build-up method is now being studied.
One of the build-up methods recently disclosed is a method for forming fine particles of an azo pigment, which is an organic pigment, using supercritical fluid or subcritical fluid (e.g., Patent Document 1). Specifically, the method comprises dissolving a pigment in supercritical fluid or subcritical fluid and allowing crystal to grow by returning the conditions of the solution to room temperature and normal pressure, thereby producing fine particles. Practicing this method involves problems that equipment capable of giving extremely high temperature and pressure near the supercritical temperature and pressure is necessary and that organic compounds are generally easily decomposed under such conditions.
The second build-up method is a method for forming fine particles using a microjet reactor, which is a micro-chemical process technology described later (e.g., Patent Documents 2, 3, and 4). The method comprises introducing a solution in which a pigment is dissolved and a medium for deposition into two opposing nozzles of a different micrometer size at high pressure (e.g., 5M Pa) by a pump, vertically injecting gas (compressed air, etc) to the area where the jet streams of the two solutions collide, and discharging the pigment suspension by the gas stream (about 0.5 m3/h). Of such methods, the method described in Patent Documents 2 and 4 are equivalent to the breakdown method, in which pigment particles are formed into fine particles by allowing pigment suspensions to collide with each other in a chamber. On the other hand, the method described in Patent Document 3 is a method for producing fine particles by spraying a pigment solution and a medium for deposition in a chamber and depositing, which can be regarded as a build-up method. The method is designed to produce particles in a narrow space of a micrometer scale and immediately discharge the particles to the outside of the reactor in order to prevent blocking of the reactor by the pigment fine particles. Although the method is suitable for preparing fine particles with a narrow particle size distribution, it has a problem that control of the contact time of solutions is difficult and thus fine control of the reaction is difficult.
The third build-up method is a method comprising gradually bringing a solution in which an organic pigment is dissolved into contact with an aqueous medium to deposit the pigment (so-called coprecipitation method (reprecipitation method)), one of the solutions containing a dispersant, thereby producing stable fine particles (Patent Document 5). Although particles of a nanometer size can be easily produced by this method, there may be fluctuation in particle size or needle particles tend to be formed upon scale-up. Therefore, although particles are measured to be single nanometer particles in a particle size measurement device, rather long needle particles are found when the particles are observed by a transmission electron microscope. Such particles are not suitable as fine particles for ink for inkjet printers for which spherical particles are desirable.
There are methods classified between the build-up method and the breakdown method. One of them is a method called conditioning, in which the particle size of coarse particles is made uniform by applying some energy. Recently, a method of conditioning by heat treatment of an organic pigment in a microreactor utilizing a concept of micro-chemical process technology (described later) is disclosed (Patent Document 6). By continuously introducing a suspension of liquid pigment precursor (a solution in which pigment with a wide particle size distribution is suspended) into a microreactor to perform heat-treatment, phase change of pigment crystal fine particles in the suspension occurs, and simultaneously, particles having a larger average particle size and a narrower particle size distribution than those of the precursor can be produced. The method has an advantage that particles with a narrow particle size distribution can be obtained, but has a disadvantage that although the precursor has a small particle size, the particle size consequently becomes large.
Patent Document 7 discloses a method of producing a surface-treated organic pigment by collision mixing of an organic pigment, an aqueous medium and a mixture containing rosin and/or a derivative of the organic pigment. To perform such collision mixing, however, use of an extremely high pressure of 30 to 300 Mpa is necessary, and therefore, considering energy consumption, such a method of collision mixing using a microscale reactor has problems of productivity and environmental load.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2002-138216
[Patent Document 2] Japanese Patent Application Laid-Open No. 2002-146222
[Patent Document 3] Japanese Patent Application Laid-Open No. 2002-155221
[Patent Document 4] Japanese Patent Application Laid-Open No. 2002-161218
[Patent Document 5] Japanese Patent Application Laid-Open No. 2003-026972
[Patent Document 6] Japanese Patent Application Laid-Open No. 2002-030230
[Patent Document 7] Japanese Patent Application Laid-Open No. 2004-175975
[Patent Document 8] Japanese Patent Application Laid-Open No. 2002-038043
[Non-patent Document 1] “Stabilization of Pigment Dispersion and Surface Treatment Technique/Evaluation”, 2001, pp. 123-224, Technical Information Institute Co., Ltd.
[Non-patent Document 2] “The Fourth Series of Experimental Chemistry” edited by the Chemical Society of Japan, vol. 12, pp. 411-488, Maruzen Co., Ltd.
[Non-patent Document 3] W. Herbst and K. Hunger, “Industrial Organic Pigments, Production, Properties, Applications, Second Completely Revised Edition”, VCH A Wiley Company, 1997, pp. 595-630
[Non-patent Document 4] H. Nagasawa et. al., “Design of a New Micromixer for Instant Mixing Based on the Collision of Micro Segments”, WILEY-VCH Verlag GmbH & Co., KGaA, Chem. Eng. Technol. 2005, 28. No. 3, p. 324-330